In a display apparatus in which a color area is relatively narrow, there is a case that operation processing of expanding (emphasizing) the saturation (hereinafter, to be referred to as “saturation expansion processing”) is carried out to image data corresponding to a display image. For example, a liquid crystal display apparatus, which uses white illumination as backlight, is narrow in a color area, compared with a wider color area of an OLED (organic light emitting diode) display apparatus in recent years. An OLED display apparatus is sometimes required to expand the color area in order to show the colors more beautifully. One technique of expanding the color area is to thicken the color of a filter for each pixel of the liquid crystal display panel. However, in this technique, the light flux of the backlight attenuates largely due to the filter so that the backlight power has to be increased to keep same brightness of the image. On the other hand, if a technique of expanding the color area by carrying out the saturation expansion processing to the image data is used, such a problem can be avoided.
In a computer containing a mobile terminal (mobile system), image data is generally given as RGB data that shows a gray scale level of each of red, green, and blue of a pixel. When the emphasis of saturation is required in a system in which the image data is given as the RGB data, the saturation expansion processing has to be carried out to the RGB data.
As one technique of carrying out the saturation expansion processing to the RGB data, a technique is known that converts the RGB data into HSV data and expands saturation in a HSV color space. The HSV color space is a color space showing a color by hue (H), saturation (S), and value (V), as shown in FIG. 1. FIG. 1 shows a cone HSV color space.
For example, Patent Literature 1 (JP 2010-79119A) discloses one example in which RGB data is converted into HSV data and the saturation is expanded in the HSV color space. In the technique disclosed in Patent Literature 1, HSV data, i.e. saturation data (S), hue data (H), and value data (V) are extracted from the display data in the RGB form. Then, the saturation data (S′) after expansion is calculated by multiplying the saturation data (S) by a saturation expansion coefficient k. Moreover, the HSV data that contains the saturation data (S′) after expansion, the hue data (H), and the value data (V) is converted into the RGB data, and the RGB data is outputted as the display data after expansion. That is, in Patent Literature 1, the operation processing is carried out in which only saturation component S is expanded while the hue component H and the value component V are saved.
However, one problem in the technique of expanding only the saturation component S while the hue component H and the value component V are saved, as disclosed in Patent Literature 1, is that the brightness declines to darken the image by saturation emphasis.
Note that Patent literature 2 (JP H06-339017A) discloses an apparatus and method of emphasizing the saturation of a color image, as another technique. The method of emphasizing the saturation disclosed in Patent Literature 2 emphasizes the saturation of the image by multiplying a difference between a maximum value I of the R value, the G value and the B value of each pixel of a color image and each of the R value, the G value and the B value by an emphasis coefficient, and setting a value obtained by subtracting the multiplying result from the maximum value I, as a corresponding one of the R value, the G value and the B value.